To improve properties of various base materials, so far, it was an accepted practice to vapor-deposit a film on their surfaces by the plasma CVD method. In the field of packing materials, it is a known art to vapor-deposit a film on the plastic base materials such as of containers by the plasma CVD method to improve gas-barrier property. For example, attempts have been made to improve gas-barrier property by vapor-depositing a film of silicon oxide on the surfaces of plastic containers such as polyethylene terephthalate (PET) bottles by the plasma CVD method by using a mixed gas of an organosilicon compound and oxygen as a reaction gas.
In recent years, however, from the standpoint of environmental problems, attention has been given to the polylactic acid which is a representative biodegradable plastic material in a variety of fields, and bottles made from the polylactic acid have been put into practical use in the field of packing materials, too. The polylactic acid bottles, however, have gas-barrier property inferior to that of the PET bottles. Therefore, attempts have been made to vapor-deposit the above-mentioned film on the polylactic acid bottles, too, to improve properties such as gas-barrier property.
Excellent gas-barrier property can be exhibited if the silicon oxide film is vapor-deposited on the PET bottles. If the silicon oxide film is vapor-deposited on the polylactic acid bottles, however, the walls of the polylactic acid bottles are thermally deformed and thermally deteriorated arousing a problem of generation of offensive odor in the bottles. The polylactic acid has a glass transition point lower than that of the PET, i.e., has an inferior heat resistance. If a silicon oxide film having gas-barrier property is vapor-deposited using a large plasma power, therefore, then the polylactic acid bottle cannot withstand the heat generated by the plasma at the time of deposition and, besides, undergoes the oxidation and deterioration due to the oxygen plasma produced during the step of vapor deposition.
Therefore, in the field of, specifically, packing, study has been forwarded to vapor-deposit films other than the silicon oxide film. For instance, patent documents 1 and 2 are proposing vapor-depositing a hydrocarbon film called diamond-like carbon (DLC) film on the inner surfaces of the polylactic acid bottles.
As compared to silicon oxide films, the hydrocarbon films can be vapor-deposited on the inner surfaces of the bottles requiring small outputs and in short periods of time without causing the polylactic acid bottles to be thermally deformed or thermally deteriorated yet providing a higher barrier property against water than that of the silicon oxide films, which are advantages. However, the hydrocarbon vapor-deposited film is hard and, besides, has no polar group; i.e., the hydrocarbon vapor-deposited film cannot be closely adhered to the polylactic acid base material such as polylactic acid bottle and tends to be easily delaminated. As described above, when it is attempted to vapor-deposit a hydrocarbon film on the polylactic acid base materials, there remains a problem of lack of close adhesion between the base material and the vapor-deposited film.
A patent document 3 proposes an art of vapor-depositing an organometal film (e.g., silicon oxide film) having an oxygen ratio that is suppressed to be low on the inner surface of a polylactic acid base material (e.g., polylactic acid bottle), and vapor-depositing a hydrocarbon film thereon. The thus vapor-deposited film of the two-layer structure has such advantages that the organometal film that is the lower layer is vapor-deposited with a small output making it possible to effectively prevent the polylactic acid base material from being thermally deteriorated or thermally deformed at the time of depositing the film and, at the same time, that the organometal film that is vapor-deposited has a low degree of oxidation and is, therefore, flexible enabling itself to be closely adhered to the polylactic acid base material. As a result, the delamination is effectively avoided and, besides, the hydrocarbon film that is vapor-deposited is allowed to exhibit its excellent properties to a sufficient degree.
However, the vapor-deposited film of the above two-layer structure involves such a defect that the reaction gas must be changed over during the step of depositing the film. Namely, changing over the reaction gas makes it difficult to maintain the continued deposition causing a decrease in the productivity and, therefore, requiring further improvements.